1. Technical Field
The embodiments herein generally relate to microelectromechanical systems (MEMS), and, more particularly, to a new differential microphone having improved frequency response and sensitivity characteristics.
2. Description of the Related Art
Determining the direction of a sound source with a miniature microphone is known in the art. Much of this technology is based on the structure of the ear of a particular parasitic fly Ormia Ochracea. Through mechanical coupling of the eardrums, the fly has highly directional hearing to within two degrees of azimuth. The eardrums are known to be less than about 0.5 mm apart such that without mechanical coupling, localization cues would be only around 50 nanoseconds. The mechanical coupling results in up to 30× amplification of this time delay, enabling the exceptionally high direction sensitivity.
The most common approach to constructing a directional microphone is provided by an apparatus comprising sound inlet ports defined by juxtaposed tubes that communicate with a diaphragm. The two sides of the microphone diaphragm receive sound from the two inlet ports. The sound pressure driving the rear of the diaphragm travels through a resistive material that provides a time delay. The dissipative resistive material must be designed to create a proper time delay in order for the net pressure to have the desired directivity.
It is important that the net pressure on the directional microphone is proportional to the frequency of the sound, and thus has a 6 dB per octave slope. The net pressure is also diminished in proportion to the distance between the ports. By reducing the overall size of the diaphragm the result is a proportional loss of directional sensitivity. It can be observed that the 6 dB per octave slope and the dependence on the distance dimension remain even in microphones devoid of the resistive material. A microphone without the resistive material is normally called a differential microphone or a pressure gradient microphone.
Directional microphones, which are commonly used in hearing aids, are normally designed to operate below the resonant frequency of the devices diaphragm. This causes the response to have roughly the same frequency dependence as the net pressure. As a result, the microphone output is proportional to frequency, as is the net pressure. The uncompensated directional output of the microphones in hearing aids exhibits a 6 dB per octave high pass filter shape. To correct for this frequency response characteristic, a 6 dB per octave low pass filter is incorporated in the hearing aid device, along with a gain stage. This yields a “flat” response. The typical microphone package incorporates a witch to allow the user to select between the two response curves.
The problem of electronically compensating for the 6 dB per octave Slope of the diaphragm response is that compensating causes a substantial degradation in noise performance. Any thermal noise introduced by the microphone itself, along with the noise created by the buffer amplifier, is amplified by the gain stage in the compensation circuit. It should be noted that the significant increase in noise is very undesirable.
Hearing aid manufacturers have found it necessary to incorporate switches on hearing aids to allow users to switch to a non-directional microphone mode in quiet environments, where the directional microphone noise proves most objectionable.
The noise inherent in conventional, directional microphones has caused hearing aid microphone designers to use a relatively large port spacing of approximately 12 mm. This is considered to be the largest port spacing that can be used while still achieving directional response at and below 5 kHz the highest frequency for speech signals.
Creating small directional microphones has been dependent upon the product of frequency and port spacing. The distance factor indicates that sensitivity of the device is reduced as its overall size is reduced.
Traditionally, compensating the output signal to achieve a flat frequency response has been accomplished electronically. Unfortunately, this has lead to the amplification of noise sources. The present invention provides a new approach to solving the aforementioned problems. By emulating a mechanical structure similar to that employed in the directionally sensitive ears of the fly, Ortnia ochracea, and a functional microphone can be made without having to rely on frequency compensation. A diaphragm not unlike that of the Ormia Ochracea ears is very well suited to silicon micro fabrication technology.
For the reasons stated above, there is a need in the art for a miniature microphone system capable of detecting a sound source location over a wide frequency range.